This invention relates generally to vacuum shrink packaging and specifically to such packaging wherein shrinkage of the packaging material over a contained product is conducted before vacuum sealing the package.
It is conventional in the field of packaging articles in flexible thermoplastic film to evacuate the interior of the package both to improve the shelf life of the packaged product and to give the package a good appearance. It is also known to improve the appearance of the vacuum sealed package by using a heat shrinkable (i.e. oriented) film as envelope for the package and subjecting the vacuum sealed package to a shrinking operation in a hot water bath or a hot air tunnel in which the plastic film is heat shrunk to bring it intimately into contact with the article therein. The present invention is particularly concerned with the heat shrinking step.
It has been conventionally preferred to carry out the heat shrinking step after the vacuumizing step by submersion of the vacuum sealed package in a hot water bath. By so doing, heat transferred to the packaging film is sufficiently rapid and uniform to provide uniform shrinkage for an attractively packaged final product. It is generally considered a processing disadvantage that wet packages must be handled, however the quality of appearance of the final package achieved with hot water shrinking is considered to offset this disadvantage.
Shrinking vacuum sealed bags with hot air convection has long been utilized, such as by the use of hot air tunnels. However, this approach has not been considered totally satisfactory, primarily because of the inability to achieve sufficient heat transfer rates to heat the packaging film in areas where the film contacts the product which acts as a heat sink. Another approach involves shrinking with hot air convection during the vacuumizing step while the packaging bag is ballooned away from a contained product, this ballooning being caused by differential rates of evacuation interior of and exterior of the bag. This approach has the disadvantage that heat transfer to the bag is adversely affected by the reduced amount of air mass in the vacuum chamber which acts as an heat transfer medium. Another approach involves shrinking in-chamber using hot air convection, as before, but wherein shrinking is conducted just prior to vacuumizing with ballooning of the bag achieved by constricting the bag mouth during heating thereby elevating pressure within the bag relative to pressure exterior of the bag.
The in-chamber prevacuumizing approach, while providing generally acceptable shrink packaging results, has the disadvantage that in a multi-chamber vacuum packaging operation each vacuum chamber must be equipped with apparatus for conducting in-chamber prevacuumizing shrinkage. For example, vacuum packaging apparatus involving a plurality of vacuum chambers is representatively shown in U.S. Pat. No. 3,958,391 issued May 25, 1976 to Kujubu wherein a plurality of vacuum chambers are moved continuously around a closed path with a vacuum packaging cycle being completed within each chamber during each revolution. During a vacuum packaging cycle within each chamber, with a loosely bagged article placed therein, the chamber is vacuumized to the desired extent causing the bag to balloon away from the enclosed article followed by extraction of air from within the bag and collapse of the bag onto the article, and then the bag is sealed in-chamber under low pressure conditions.
Representative examples of prevacuum shrinking by hot air convection are disclosed in U.K. patent application No. GB2094745A published Sept. 22, 1982 to Gianelli at. al. A package is formed by placing a loaded bag of a heat shrinkable material into a vacuum chamber and operating hot air fans to circulate air over heaters within the closed chamber to apply heat to the bag which causes air trapped within the container by bag mouth restriction means to expand and balloon the container away from contact with the product so that further forced convection heat is more readily able to shrink the bag into contact with the product. The bag is then punctured in its neck area, the chamber evacuated and the bag finally sealed before venting and opening of the chamber. The bag mouth restricting means representatively comprise a resilient leaf valve biased against a counter-support to releasably hold the bag mouth closed while permitting venting of any excessive pressure build-up in the bag.